A historical interlude Niels Bohr 18851963

Niels Bohr was born on 7 October 1885 in Copenhagen, Denmark. His father, Christian, was then Privatdozent, and later Professor of

Physiology at the University of Copenhagen. His mother was Ellen Adler.

Bohr did well in school without being brilliant. He excelled in physical education and was a good soccer player, although not quite as good as his brother Harald, who won a silver medal playing for Denmark in the 1908 London Olympics.

Bohr entered the University of Copenhagen in 1903 to study Physics. In 1905, while only a second year student, he decided to try for the prize essay of the Royal Danish Academy, on the subject of 'the vibration of liquid jets'. He constructed his own apparatus, and impressed the judges to such an extent that he was awarded a Gold Medal in January 1907.

Bohr received his master's degree in 1909, and his doctorate in 1911. A grant from the Carlsberg Foundation enabled him to go to Cambridge in the same year, to study under Sir J.J. Thomson. He brought with him a copy of his thesis. Thomson was preoccupied with his own work and gave little of his time to students. In one of his letters to Harald, Niels complained that his manuscript lay unread under a pile of papers on Thomson's desk. Within less than a year

Bohr left Cambridge to join Ernest Rutherford's research team in Manchester.

Rutherford's laboratory was a hive of activity. Rutherford himself had just published what has since been recognised as one of the classic papers in the history of physics, showing that the mass of an atom was concentrated in a tiny nucleus. In another letter to Harald, Niels wrote: 'You can imagine it is fine to be here, where there are so many people to talk with.and this with those who know most about these things...'

Bohr had married Margarethe Norlund in 1912, who accompanied him to Manchester and was expecting their first child when the Bohrs returned to Denmark in 1916. They had six sons, one of whom, Aage, followed his father into physics, and eventually into the ranks of Nobel Prize winners (in 1975).

Soon after his return from Manchester, Bohr was appointed to the Chair of Theoretical Physics at the University of Copenhagen. In 1917 he was elected to the Danish Academy of Sciences, and obtained the necessary backing to establish the Institute of Theoretical Physics in Copenhagen. He became its director from the opening in 1921, a post which he held for the rest of his life.

The institute soon attracted physicists from all over the world. The list reads like a who's who in physics at the time: Lise Meitner (Austria), Georg Hevesy (Hungary), Peter Kapitza, Hendrick Casimir, George Gamow and Lev Landau (Russia), Paul Dirac (England), Werner Heisenberg (Germany), Robert Oppenheimer (USA), Kazuhiko Nishijima (Japan), Albert Einstein.. The atmosphere was one of great excitement. The structure of the atom was being unravelled and the fascinating laws which govern the world of the very small was being explored.

In his acceptance speech at the Nobel Prize ceremony in 1922, Bohr was able to say: 'We not only believe in the existence of atoms, but we believe that we have an intimate knowledge of the constituents of the individual atom. All physical and chemical properties of substances are now clear.'

Discussion of physics went on most nights until it was very late, but there still was time for games and banter. Bohr, who liked Westerns, produced a theory as to why the hero shoots faster even-through the villain draws first, and they tried out the theory with toy pistols. Bohr's theory was that the premeditated action of the villain was not as fast as the hero's automatic reaction. Bohr, who always played the hero, was usually faster, but it was not clear if this was due to Bohr's natural athleticism or the more profound reasons given by the theory.

One night, when returning from a party, Gamow, Casimir and Bohr tried to climb the wall of the Bank of Copenhagen building. The police, on arriving, took no action — 'Oh, it is only Professor Bohr!'

There was a horseshoe above the door of the institute. When asked if he believed in luck, Bohr replied: 'Certainly not, but they say it does bring luck even to those who don't believe in it'.

Considerable financial support for the Danish Royal Academy came from the Carlsberg brewery. There was no problem with accommodation, as Carl Jacobsen, whose father had founded the brewery, had specified in his will that his mansion in Copenhagen was to be used as a residence by the 'most distinguished Danish scientist, at that time, Niels Bohr.

Bohr was extremely exact and meticulous in every word he wrote or spoke. In fact his colleagues and students invented the term 'Bohrspeak, defined as a closely knit language in which every word counted and which required very careful listening. Bohr's sentences were carefully constructed to express his thinking. As he said himself: 'You can never express yourself more clearly than you can think.'

On 11 February 1939, two Austrian physicists, Otto Frisch (1904-1979) and Lise Meitner (1878-1968), published a paper in Nature, in which they interpreted the results of earlier experiments by Meitner and Otto Hahn (1879-1968) as a new process which they called nuclear fission. When uranium is bombarded by neutrons, it breaks into 'two nuclei of roughly equal size'. The reaction involves the uranium nucleus, the tightly knit core of the atom, hitherto considered impenetrable. Much energy is released in the process together with more neutrons, which can in turn trigger the reaction in neighbouring uranium nuclei. Bohr had been informed of the results in advance, and announced their conclusions at a meeting of the American Physical Society, in Washington DC, on 26 January 1939.

Following the outbreak of World War II and the occupation of Denmark by Germany, Bohr received a visit from his former German student Werner Heisenberg. There is no historical record of what transpired during that visit. It is possible that Heisenberg told Bohr that Germany was on the point of using fission in uranium to make an atomic bomb. Perhaps he was trying to pick Bohr's brains on the critical mass of uranium 235 required to sustain a chain reaction. A play by Michael Frayn, entitled 'Copenhagen', which opened in London's West End in 2001, puts forward the above two alternatives, as well as a third conjecture, that Heisenberg came to warn Bohr of the danger of remaining in Denmark, and to urge him to escape.

It was later revealed at the Nuremberg trials that it was the original intention to arrest Bohr immediately after the day of the occupation of Denmark, 29 August 1943. It was decided to postpone the order for about a month, when such an arrest 'would attract less attention.'

On 29 September 1943, as it later transpired in the nick of time, Bohr and his family were smuggled by fishing boat to Sweden. On the very next day Bohr, who had already arranged the reception of many refugees to Sweden, met the Secretary of State and King Gustav V of Sweden to try to arrange the acceptance of more ships of Jews 'for internment in Sweden'. Unfortunately, at this stage, the Swedes were powerless to comply.

From Sweden, Bohr and his son Aage were flown to England in the bomb bay of an RAF Mosquito, nearly dying from the cold and oxygen starvation in the process. Apparently the flying helmet was too small to fit Bohr's rather large head, resulting in a faulty connection in the oxygen supply tube, so that he was actually unconscious when the plane landed. On his arrival in England he heard of the development of fission research in the US.

Bohr had believed that the rare U-235 isotope of uranium, which made the chain reaction possible, was too difficult to separate in quantities large enough to make an atomic bomb. But now news had come that Peirls and Frisch in Birmingham had shown that the critical mass of U-235 was about 1 kg, and not 1 ton, as previously believed. The US had imported 100 tons of uranium ore from the Belgian Congo. The implications were clear — the Americans believed that the manufacture of a bomb was possible.

Bohr immediately wrote to Winston Churchill: 'This project will bring either disaster or benefit to mankind on a scale hitherto unimaginable.' He tried to persuade Churchill to take the development of nuclear power seriously, to ensure that it would serve mankind as a whole, and renewed his request that the development of an atomic bomb be dropped. Churchill was not impressed, and it was clear to Bohr that he was making little impact.

Shortly afterwards the Bohrs sailed secretly to the Los Alamos Atomic Laboratories in the US, where Bohr became known as 'uncle

Nick' as his security name was Nickolas Baker. He saw the enormous plant for the separation-of-uranium part of the Manhatten project under his former student Robert Oppenheimer. The great energy derived from the atomic nucleus was clearly to be used as a weapon of war and nothing could be done to stop the project. Bohr admitted: 'They don't need me to make an atomic bomb.'

When World War II ended in Europe on 8 May 1945, Bohr's primary concern was to prevent a nuclear arms race. Nuclear power could lead to the greatest boom for mankind, but also to man's greatest disaster. Bohr travelled to London to meet Churchill on 16 May 1944 to try to impress on him that control of nuclear weapons

Jumbo atomic device for the 'Trinity test'.

was vital to world security, but met with a frosty reception. Churchill clearly distrusted him. A meeting with Roosevelt on 26 August 1944 was considerably more amiable, but still he could not stop military plans to use atomic bombs against Japan. Two bombs were exploded on Hiroshima and Nagasaki on 6 and 9 August 1945.

At the end of August 1945 Bohr returned to Denmark and continued where he had left off. Unpretentious as ever, he rode every day to the institute on his bicycle. The year 1952 saw the establishment of the centre of nuclear research (CERN) in Geneva, with Niels Bohr as its first chairman. Bohr devoted the rest of his life to promoting peaceful uses of atomic energy. He died on 18 November 1962 in Copenhagen.